Getting a person wet before casting a lightning spell is NEVER going to
increase damage because:

1) Humans are already 72% water. Splashing a little bit of water on a
person isn't going to make enough of a difference to double the damage.

2) water is not very conductive anyway: it's the mineral solids
dissolved in the water (or in this case, the solids in the human body)
that make it conductive, so getting a person wet isn't going to change
their conductivity because it's the PERSON part that's conducting most
of the electricity. Read this handy little tidbit from
http://en.wikipedia.org/wiki/Water
"A common misconception about water is that it is a powerful conductor
of electricity, with risks of electrocution explaining this popular
belief. *Any electrical properties observable in water are due to the
ions of mineral salts and carbon dioxide dissolved in it.* Water does
self-ionize where two water molecules become one hydroxide anion and one
hydronium cation, but not enough to carry enough electric current to do
any work or harm for most operations."

3) Even if water WERE insanely conductive or susceptible to electricity,
it would only make it Easier for the lightning to travel to ground and
do less damage. When things are highly conductive, electricity can pass
through them with very little difficulty (ie, when a properly grounded
lightning rod is struck it allows the electricity to flow safely to
ground with only minor wear and tear to the rod and cable). When
something does Not conduct electricity well - such as WOOD - it is much
likelier to sustain damage from a lightning strike - for example, a
nasty old tree being struck by lightning is likely to explode. Even a
healthy tree will often have a strip of bark blasted off its side from
the point of the strike to the ground.

4) WATER AND LIGHTNING ARE NOT OPPOSING ELEMENTS. Lightning ONLY does
extra damage against Earth-aligned creatures, and those that are
specifically weak to lightning! Period!